Acoustic surface wave device with reduced rf feedthrough

ABSTRACT

An acoustic surface wave device of the type having a launching transducer and a receiving transducer disposed in relatively close proximity on a substrate of piezoelectric material wherein the dynamic range of the device is increased by the reduction and elimination of spurious responses resulting from undesired, RF feedthrough signals due to direct coupling between the transducers, the device incorporating a radiating element or structure disposed in the vicinity of the launching transducer and fed RF input signal energy in phase opposition to &#39;&#39;&#39;&#39;buck&#39;&#39;&#39;&#39; out the leakage component at the receiving transducer because the bucking signal energy generated by the radiating element is seen at the receiving transducer to be of equal magnitude and out of phase with the directly coupled leakage component thereat.

United States Patent 91 Weglein 11 3,845,418 [451 Oct. 29, 1974 ACOUSTICSURFACE WAVE DEVICE WITH REDUCED RF FEEDTHROUGH Rolf D. Weglein, LosAngeles, Calif.

Hughes Aircraft Company, Culver City, Calif.

Filed: Jan. 2, 1974 Appl. No.: 430,360

US. Cl 333/30 R, 3l0/9.8, 333/72 Int. Cl H03h 9/26, H03h 9/30, l-lOlv7/00 Field of Search 333/30 R, 30 M, 72; 310/8,

Inventor:

Assignee:

References Cited UNITED STATES PATENTS 4/1971 De Vries et al 333/723/1973 Adler 333/30 R X [5 7 ABSTRACT An acoustic surface wave device ofthe type having a launching transducer and a receiving transducerdisposed in relatively close proximity on a substrate of piezoelectricmaterial wherein the dynamic range of the device is increased by thereduction and elimination of spurious responses resulting fromundesired, RF feedthrough signals due to direct coupling between thetransducers, the device incorporating a radiating element or structuredisposed in the vicinity of the launching transducer and fed RF inputsignal energy in phase opposition to buck out the leakage component atthe receiving transducer because the bucking signal energy generated bythe radiating element is seen at the receiving transducer to be of equalmagnitude and out of phase with the directly coupled leak- 8 Claims, 7Drawing Figures PAIENTEUnmza-mu SNEEII-OFZ 29 v Fig. l.

ACOUSTIC SURFACE WAVE DEVICE WITH REDUCED RF FEEDTHROUGH BACKGROUND OFTHE INVENTION The background of the invention will be set forth in twoparts.

1. Field of the Invention This invention relates generally to acousticsurface wave devices and more particularly to such devices incorporatingr.f. feedthrough reducing arrangements.

2. Description of the Prior Art The availability for beneficial use ofacoustic waves that propagate along a boundary surface is well known.This phenomenon was first described by Lord Rayleigh in an articleentitled On Waves Propagating Along the Plane Surface of an ElasticSolid, Proceedings, London Mathematics Society, Vol. 17, pages 4-l 1,Nov. 1885. Devices utilizing such acoustic surface waves have theadvantage of allowing easy access at all times to the propagatingacoustic energy, to sample it, to modify and to interact with it.

The typical particle motion associated with acoustic surface waves iselliptical retrograde and the amplitude decays exponentially with depthinto the body of the acoustic surface wave-propagating medium. Thus,acoustic surface waves are localized at the surface of solids, typicallyof piezoelectric material such as quartz and lithium niobate (LiNbO andbismuth germanium oxide (Bi Ge for example.

The advantageous use of acoustic surface wave techniques (elastic waves)has probably been most pronounced in devices operating at very highfrequencies (VHF), ultra high frequencies (UHF), and microwavefrequencies. It has been found that such operation provides excellenttransmission characteristics and relatively low propagating velocitiesof approximately 5 orders of magnitude less than that of the speed oflight or that of electromagnetic waves. As an example, an elastic waveresonator operating at a given frequency is typically 100,000 timessmaller than an electromagnetic wave resonator for the same frequency,and the low propagation losses of the acoustic medium allows delay timesof about I00 times that possible with lowloss electromagnetic waves.

The basic building block of all surface wave devices is the acousticsurface wave delay line which includes spaced transducers disposed on asurface wave supporting medium. The transducers provide transitions fromnormal electric circuitry into the acoustic do main.

Transducers designed for operation with piezoelectric media, mostcommonly are of the interdigital type consisting of a series ofconductive electrodes that form a pattern which is disposed on thepiezoelectric substrate surface. Such transducers are two-terminaldevices having two separate arrays of metal strips resemblinginterleaved fingers, and convert electrical signals into acousticsurface waves, and also are capable of converting such elastic waveenergy incident thereon into electrical signals. In the case of an inputsignal transducer, an incoming electrical signal is converted by thetransducer into a time-dependent spatially varying-electrical fieldpattern which in turn generates and launches an acoustic surface wavedirectly on the substrate through the piezoelectric action of thesubstrate material.

In the past, spurious responses in acoustic surface wave delay deviceshave occurred as a result of undesired r.f. energy feedthrough couplingbetween the launching and receiving transducers. This type of spurioussignal has no delay since it travels in space between the transducers atthe speed of light and not through the acoustic medium which has a muchslower propagation velocity characteristic.

Several schemes have been developed over the years in an effort to atleast reduce such undesired electromagnetic intertransducer coupling.One such scheme is to balance opposite polarity signal components at thelaunching transducer, as exemplified in US. Pat. No. 3,573,763. In thisapproach, at least one of the transducers, and preferably both, arecoupled to their respective source and load by circuitry which causesthe signals developed across the transducers to be balanced with respectto ground, both terminals of the launching transducers being aboveground potential. Thus, undesired feedthrough signal energy developed atthe receiving transducer has two components of equal but opposingvalues, and hence cancelling polarities.

Another scheme described in the foregoing patent utilizes the shieldingeffect of a plane or planes of fixed reference potential disposedbetween a launching and receiving transducer. The shield element isoperated at ground potential and has no signal currents thereat in orderto minimize the potential of these shielding electrodes that otherwisemight create coupling fields to the electrodes in adjacent transducers.This type configuration can be very effective for relatively long delays(greater than 1 usec) where sufficient space is available betweentransducers to produce adequate shielding. It should therefore beevident that in applications having relatively short delays (less than Iusec), adequate space is generally not available to introduce shieldingfixtures. As an example. a time delay of 0.1 usec spans a distance of0.3 mm (or 12 mils). The problem is that effective shielding must notonly reduce the direct r.f. feedthrough component to tolerable levels,but must simultaneously leave .the performance of the surface wavedevice unaltered. Thus, as the desired delay decreases, the shieldingstructure must be brought into closer proximity to the transducers,where eventually, interaction with the transducers may impair theelectrical perfomance of the device.

SUMMARY OF THE INVENTION In view of the foregoing factors and conditionscharacteristic of the prior art, it is a primary object of the presentinvention to provide an improved acoustic surface wave device withreduced r.f. feedthrough.

Another object of the present invention is to provide a very simple yetefficient construction for an acoustic surface wave device with reducedfeedthrough, having relatively short delays.

Yet another object of the present invention is to provide an acousticsurface wave device with reduced direct r.f. feedthrough andincorporating an ungrounded bucking electrode disposed adjacent alaunching transducer.

In accordance with an embodiment of the present invention, an acousticsurface wave device includes a substrate of piezoelectric materialcapable of propagating acoustic surface wave energy and transducer meansincluding at least a launching transducer for converting an electricalinput signal into acoustic surface waves propagating along thesubstrate. The transducers may also include an associated receivingtransducer for converting the acoustic surface waves incident thereoninto electrical signals with a delay essentially determined by thepropagating velocity characteristic of the substrate material and thedistance between the launching and receiving transducers, the distancebeing sufficiently small that a direct r.f. feedthrough signal couplingexists between the transducers. The device further includes buckingmeans with a phase shifting electrical arrangement coupled to theelectrical input of the launching transducer and with a conductivebucking electrode structure disposed adjacent the launching transducerand coupled to the electrical arrangement for coupling directly to thereceiving transducer a bucking signal of equal magnitude and oppositephase than that of the direct r.f. feedthrough signal.

The features of the present invention which are believed to be novel areset forth with particularity in the appended claims. The presentinvention, both as to its organization and manner of operation, togetherwith further objects and advantages thereof, may best be understood bymaking reference to the following description taken in conjunction withthe accompanying drawings in which like reference characters refer tolike elements in the several views.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a schematic plan view of anembodiment of the present invention;

FIG. 2 is a graphical illustration of electrical signals incident on thereceiving transducer of FIG. 1;

FIG. 3 illustrates typical oscilloscope patterns of electrical outputsignals from the receiving transducer of FIG. 1, one without a buckingsignal applied and one with such a signal applied thereto;

FIG. 4 is a partial schematic plan view of an alternate launchingtransducer and bucking electrode arrangement in accordance with anotherembodiment of the invention;

FIG. 5 is a partial schematic plan view of another alternate launchingtransducer and bucking electrode arrangement, in accordance with stillanother embodiment of the present invention;

FIG. 6 is an elevational view illustrating yet another embodiment of thepresent invention; and

FIG. 7 is a schematic illustration of still a further embodiment of thepresent invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT:

Referring now to the drawings and more particularly to FIG. 1, there isshown an embodiment of the invention in the general form of a simpledelay line 11 that includes an elongated slab or substrate 13 ofpiezoelectric material such as, for example, y-z LiNbO on a planarsurface 15 on which are disposed by any conventional means a launchingelectro-acoustic transducer 17 and a receiving electro-acoustictransducer 19.

The launching transducer 17 is a two-terminal device of the interdigitaltype across which terminals 21 and 23 is coupled a source 25 of r.f.input signal energy, lower terminal 23 being connected to a commonreturn or ground. Also connected to the source 25 is a phase shiftingelectrical arrangement 27 including a phase shift network 29 and anattenuator 31, the latter being connected to a conductive buckingelectrode structure 33 that is disposed, in this embodiment, on thesubstrate surface 15 adjacent the launching transducer 17.

Acoustic surface waves launched along the substrate 13 toward thereceiving interdigital transducer 19 in response to r.f. input signalenergy, propagate to and are incident on the receiving transducer whichconverts the'surface waves into electrical signals appearing at itsterminals 35 and 37 and the load 39 connected thereto. The receivedelectrical signals are delayed an amount essentially determined by thepropagating velocity characteristic of the substrate material and by thedistance between the launching and receiving transducers.

In the case where relatively short delays (less than 1 11sec) arerequired, the distance separating the transducers is relatively shortand a significantly high level of r.f. feedthrough signal will becoupled either capacitively, inductively, or both, from the launchingtransducer 17 to the receiving transducer 19. This directly coupledfeedthrough signal represents a serious spurious response having nodelay and which, if not reduced, will decrease the dynamic range and/orsignal purity of the device 11. The magnitude and phase of thefeedthrough leakage signal, V relative to that of the signal applied tothe launching transducer V is illustrated in FIG. 2. The r.f.feedthrough signal is also illustrated at trace a in FIG. 3 as thesmaller spurious response 41 with no delay as compared to the desireddelayed output signal 43.

In accordance with the invention, a portion of the r.f. input signal isprocessed in the phase shifting arrangement 27 and is applied to thebucking electrode structure 33 to be directly coupled to the receivingtransducer 19 so that there is incident thereat a bucking signal ofequal magnitude but opposite phase to the undesired feedthrough signal,as illustrated by the magnitude and direction of the vector labeled V,,in FIG. 2. With this diametrically opposed signal introduced inconjunction with the feedthrough signal, there occurs a cancellation ofthese two directly coupled, no delay signals without impairment of thedesired electrical performance of the device. The desired results ofthis cancellation is shown in oscilloscope trace b of FIG. 3. The phaseshift network 29 and the attenuator 31 are of conventional design andmay be adjustable so that maximum cancellation of the spuriousfeedthrough signal may be obtained by monitoring the output signal ofthe receiving transducer while varying the amount of phase shift and themagnitude of the bucking signal.

FIGS. 4, S and 6 illustrate differing launching transducer-buckingelectrode structure configurations 45, 47 and 49, respectively, inaccordance with the present invention. In FIG. 4, the bucking electrodestructure 33' is in the form of a hollow rectangular or square structuresurrounding the launching transducer 17, while the bucking structure 33"in FIG. 5 is disposed at the side of the transducer 17, and mounted aselectrode 33" above the substrate surface 15 on spaced insulating strips51 downstream of the transducer 17 in FIG. 6. In all cases, the phaseshift and magnitude of the bucking signal radiated by the buckingstructure is chosen to cancel the spurious, directly coupled, r.f.feedthrough signal appearing at the receiving trans ducer l9.

Referring now to FIG. 7, there is shown another embodiment 53 of theinvention. Here, an input r.f. signal source 55 is connected to aprimary coil 57 which couples the signal to a center tapped, balanced,secondary coil 59. The input signal developed across the portion of thecoil 59 between the center tap 61 and the upper end 63 thereof isconnected to a launching transducer 65 disposed on a piezo-electricsubstrate 67, the center tap and the lower terminal of the transducerbeing returned to ground as illustrated in the figure. The opposite end69 of the coil 59 develops a bucking signal of equal magnitude andopposite phase to the input signal coupled to the launching transducer65 and is connected to a bucking electrode structure 7] so that alldirectly coupled r.f. feedthrough energy incident on a receivingtransducer 73 is cancelled out. ln this embodiment, the balanced coilarrangement provides the required opposing phase relationship while thepositioning and orientation of the bucking electrode 71 provides therequired equal magnitude parameter to cause cancellation of the directlycoupled spurious signal.

From the foregoing, it should be evident that there has been describedan improved acoustic surface wave device which incorporates simple, yeteffective means for cancelling a significant portion of any directlycoupled r.f. feedthrough signal energy incident on a receivingtransducer. It should also be understood that the materials andstructural elements used in fabricating the various embodiments of theinvention are not critical and any material and elements or circuitrygenerally considered suitable for a particular purpose or applicationmay be utilized.

Accordingly, it should be realized that although the present inventionhas been shown and described with reference to particular embodiments,various changes and modifications are possible within the knowledge ofthose skilled in the art and are therefore deemed to lie within thespirit, scope and contemplation of the invention.

What is claimed is:

1. An acoustic surface wave device, comprising:

a substrate of piezoelectric material capable of propagating acousticsurface wave energy; 7 transducer means including at least a launchingtransducer for converting an electrical input signal into acousticsurface waves propagating along said substrate, and including anassociated receiving transducer for converting said acoustic surfacewaves incident thereon into electrical signals with a delay essentiallydetermined by the propagating velocity characteristic of the substratematerial and by the distance between said launching and receivingtransducers, said distance being sufficiently small that a direct r.f.feedthrough signal coupling exists between said transducers; and buckingmeans including a phase shifting electrical arrangement coupled to theelectrical input of said launching transducer and also including aconductive bucking electrode structure disposed adjacent said launchingtransducer and coupled to said electrical arrangement for couplingdirectly to said receiving transducer a bucking signal of equalmagnitude and opposite phase than that of said direct r.f. feedthroughsignal. 2. The device according to claim 1, wherein said phase shiftingelectrical arrangement includes a phase shift network and an attenuatorcoupled between said electrical input of said launching transducer andsaid bucking electrode structure.

3. The device according to claim 2, wherein said phase shift network andsaid attenuator are adjustable.

4. The device according to claim 1, wherein said bucking electrodestructure is an elongated conductive strip disposed on said substratesurface between said launching and receiving transducers.

5. The device according to claim 1, wherein said bucking electrodestructure is a hollow rectangular conductive strip disposed on saidsubstrate surface and surrounding said launching transducer.

6. The device according to claim 1, wherein said bucking electrodestructure is an elongated conductive strip disposed on said substrate atone side of said launching transducer.

7. The device according to claim 1, wherein said bucking electrodestructure is a relatively wide elongated conductive strip disposed ininsulative relationship above said substrate surface.

8. The device according to claim 1, wherein said phase shiftingelectrical arrangement includes a balanced center tapped coil acrosshalf of which is coupled said launching transducer and across the otherhalf of which is coupled said bucking electrode structure.

1. An acoustic surface wave device, comprising: a substrate ofpiezoelectric material capable of propagating acoustic surface waveenergy; transducer means including at least a launching transducer forconverting an electrical input signal into acoustic surface wavespropagating along said substrate, and including an associated receivingtransducer for converting said acoustic surface waves incident thereoninto electrical signals with a delay essentially determined by thepropagating velocity characteristic of the substrate material and by thedistance between said launching and receiving transducers, said distancebeing sufficiently small that a direct r.f. feedthrough signal couplingexists between said transducers; and bucking means including a phaseshifting electrical arrangement coupled to the electrical input of saidlaunching transducer and also including a conductive bucking electrodestructure disposed adjacent said launching transducer and coupled tosaid electrical arrangement for coupling directly to said receivingtransducer a bucking signal of equal magnitude and opposite phase thanthat of said direct r.f. feedthrough signal.
 2. The device according toclaim 1, wherein said phase shifting electrical arrangement includes aphase shift network and an attenuator coupled between said electricalinput of said launching transducer and said bucking electrode structure.3. The device according to claim 2, wherein said phase shift network andsaid attenuator are adjustable.
 4. The device according to claim 1,wherein said bucking electrode structure is an elongated conductivestrip disposed on said substrate surface between said launching andreceiving transducers.
 5. The device according to claim 1, wherein saidbucking electrode structure is a hollow rectangular conductive stripdisposed on said substrate surface and surrounding said launchingtransducer.
 6. The device according to claim 1, wherein said buckingelectrode structure is an elongated conductive strip disposed on saidsubstrate at one side of said launching transducer.
 7. The deviceaccording to claim 1, wherein said bucking electrode structure is arelatively wide elongated conductive strip disposed in insulativerelationship above said substrate surface.
 8. The device according toclaim 1, wherein said phase shifting electrical arrangement includes abalanced center tapped coil across half of which is coupled saidlaunching transducer and across the other half of which is coupled saidbucking electrode structure.